When a survey instrument, a measuring instrument, etc. are used for surveying purpose, it is necessary to make compensation for tilting of the reference plane of the survey instrument or the measuring instrument or to compensate deviations from the verticality of an optical axis.
In the past, compensation has been automatically performed as follows: A pendulum such as a lens or a prism is suspended by 2 or 3 suspension lines. When main unit of the survey instrument or the measuring instrument is tilted, the pendulum is braked by a braking mechanism such as a magnetic braking mechanism to automatically compensate optical path.
As one of the methods to detect tilting of reference plane of main unit of the measuring instruments, reflection on a free liquid surface is utilized.
In this method, a light beam is projected onto the free liquid surface, and any change in the optical axis of the reflected light is detected by a photodetector. When mercury is used as a liquid having free liquid surface and the light beam is projected perpendicularly to the free liquid surface, it is possible to obtain reflection angle of the same sensitivity to tilting of the liquid surface in all 2-dimensional directions, and tilting of the reference plane can be detected.
However, the liquid such as mercury as described above is not very practical to use in terms of both cost and safety, and transparent liquid such as silicone oil is used in practical application. When the transparent liquid is used, total reflection is utilized for the purpose. Because of critical angle between liquid and air, in order to achieve total reflection of light beam on liquid surface, it is necessary to project the light beam onto the free liquid surface at an incident angle .theta. to match the above critical angle. In conventional type tilt detecting device utilizing the free liquid surface, the light beam is irradiated to the free liquid surface at a predetermined angle.
When a light beam is irradiated to the free liquid surface at a predetermined angle, the change of reflection angle of the reflected light relating to different biaxial directions to the tilting of the liquid surface is not uniform. Therefore, in the tilting detecting device utilizing a free liquid surface, measures must be taken to take into account the non-uniform change of reflection angle in biaxial directions. For this reason, light beams with two different optical axes are projected at a predetermined angle to the free liquid surface, and the reflected light beams are received by photodetectors. Light receiving position in only one direction is detected by each photodetector, and by the change of the light receiving positions of the photodetectors, tilting to the two optical axes is detected. From the tilting of the two optical axes thus detected, tilting of reference planes of survey instrument, measuring instrument, etc. to horizontal plane are calculated, and compensation is performed based on the results of the calculation.
In the former example of the conventional type device described above, however, a pendulum is suspended and this leads to more complicated structure. Also, the suspension of the pendulum during assembling of the device is not easy, and adjustment is also not simple. Further, the change over time of suspension lines causes change in its length, and it is very difficult to maintain the accuracy. Because a special braking device is required for braking the pendulum, this also makes the structure more complicated. Further, suspension structure of the pendulum is very delicate and is susceptible to shock.
In the latter example of the conventional type device, utilizing total reflection on a free liquid surface, assembling and adjustment are simple, and there is no change over time because no suspension line is used. Because the liquid is used in closed condition, the device has high shock-resistance and high resistance to environmental conditions. Because liquid is used, braking can be performed by utilizing viscosity of the liquid, and no braking device is required. Thus, the problems in the vertical direction tilting automatic compensator using a pendulum have been overcome by the latter device.
However, because light beams with two different optical axes are irradiated to the free liquid surface at predetermined angles, projection system of light beams is divided into two optical systems, and this leads to the more complicated arrangement of the device.
To solve the above problems, the compensator of the present invention detects tilting of reference plane or performs automatic compensation of vertical line by monoaxial optical system only, utilizing total reflection of free liquid surface.